Freeze the resolved interpreter for child processes

[CarloLucibello]

When PythonCall resolves its interpreter via CondaPkg, each child process that loads
PythonCall re-resolves the same CondaPkg environment from scratch. Spawn several at once —
e.g. Distributed workers for parallel data loading (MLUtils DataLoader(...; num_workers=N)
over a PythonCall-backed dataset) — and they serialize on CondaPkg's lock, spamming
CondaPkg: Waiting for lock to be freed and redoing work the parent already did.

PythonCall already has the fix, but gates it behind CI=true and flags it as a hack
(src/C/context.jl, v0.9.35):

# HACK: If we are using CondaPkg, prevent child processes from using it by explicitly
# setting the executable. ...
# A better solution may be to use some environment variable to "freeze" CondaPkg in
# child processes.
# Only done when CI=true since it's a hack.
if (get(ENV, "CI", "false") == "true") && (CTX.which === :CondaPkg)
    ENV["JULIA_PYTHONCALL_EXE"] = CTX.exe_path::String
end

Since JULIA_PYTHONCALL_EXE is exactly the "freeze" env var the comment asks for (child
processes inherit ENV, and getpref_exe reads it), the same line solves the general case.

Proposal

Set JULIA_PYTHONCALL_EXE whenever CTX.which === :CondaPkg, not only under CI — ideally
behind a preference (e.g. freeze_child_processes, default true) so it can be disabled.
get! already leaves a user-set value untouched.

Workaround

A downstream package can do this itself after init:

if PythonCall.C.CTX.which === :CondaPkg
    get!(ENV, "JULIA_PYTHONCALL_EXE", PythonCall.python_executable_path()::String)
end

I've done this in JuliaGenAI/HuggingFaceDatasets.jl#64